The output signal levels of most RF circuits, particularly active circuits, have a relatively smooth variation over the operational frequency band, whereas subsequent circuits, or amplifier stages, often need, or require, a nearly constant input signal level. For example, the gain of high power RF transistors generally varies over the frequency band. This can cause problems in a multi-stage RF amplifier where a first power transistor drives a second power transistor. If the transistor to be driven can only operate within certain upper and lower limits of drive power, and the output power of the driver transistor falls outside of these limits, then the driven amplifier will not function properly over the desired frequency band. A passive equalizer circuit with low input and output return loss can remedy this problem by attenuating the output power of the driver transistor over a portion of the operating band, while providing less attenuation over the remainder of the band, including a frequency with near zero loss.
Amplitude equalizers can be designed to operate over wide band-widths at low power levels. See D. J. Mellor, On the Design of Matched Equalizer of Prescribed Gain versus Frequency Profiles, MWSYM 77, at pp. 308-311; W. H. Ku et al., Microwave Octaveband GaAs FET Amplifiers, IEEE Int. Microwave Syrup. Dig. Tech. Papers, at pp. 69-72 (1975).
These circuits are relatively complex because of the wide bandwidths involved; the fact that their power capacity is insufficient to be used with microwave power transistors is not important, for the circuits are satisfactory for their intended use.
By contrast, high power amplifiers (&gt;5 watts) generally operate over restricted bandwidth (15% or less). Equalizers for these amplifiers must be capable of passing and dissipating considerable amounts of power, must have very well matched input and output impedances, and must not reduce the minimum power of the amplifier.
It is, therefore, an object of this invention to provide a simple equalizing circuits, consisting of one basic element, that can handle up to kilowatts of peak power and many watts of average power that has well matched input and output impedances, and can have zero loss at some point(s) in the operating frequency band.